It is often desirable to selectively detect the intrusion of objects into a predetermined region. If the detected intrusion or attempted intrusion is indiscriminant, false alarms can be produced by inconsequential events. Thus a selective detector is desirable. One useful basis for discrimination is the degree of force required to trip the detector.
It is well known to use a limit switch to detect the movement of an object to a preset position. However, where an extended detection region or zone is desired optical detectors for detecting intrusion of an article into a light beam are often used. Typically the light beam is a straight path and not suitable for application to a curved region. Also the light beam sensors generally are not discriminating since the light beam can be blocked by a piece of paper or an insect as well as something which exerts a significant amount of force.